1. Field of the Invention
This invention is directed to wellbore cementing systems; plug set release systems; plug containers; and swivel equalizers for well tools and apparatuses.
2. Description of Related Art
During the construction of oil and gas wells a bore is drilled into the earth. Casing is then lowered down the bore and the annular space between the outside of the casing and the bore is filled with cement. The casing is generally held centrally in the bore by centralizers which are mounted on the casing at spaced intervals therealong. Typically, a non-return valve known as a "float valve" is mounted on or adjacent the bottom of the casing. During a typical cementing operation the annular space is first cleared by pumping circulating fluid down the inside of the casing and allowing it to flow upwardly through the annular space. When the annular space is clean a bottom plug is placed in the casing. The bottom plug is pumped ahead of cement to separate the cement from drilling mud and other wellbore fluids. The bottom plug typically has wipers of elastomeric material to wipe mud from the casing so it does not contaminate the cement. When the plug reaches float equipment at the bottom of the casing string, a fluid pressure differential created across the plug ruptures a rupturable member of the plug and allows the cement to flow down the casing, through the plug and float equipment, and up into an annular space between the casing and the wellbore. When the cement flow ceases, a top cementing plug is released from the plug container. The top plug follows the cement and reduces contamination or channeling of the cement by drilling mud that is used to displace the cement column down the casing and into the annular space. The top cementing plug sealingly contacts the bottom cementing plug at the float equipment to effect a shut off of fluids being pumped into the casing. The return flow of cement back into the casing in inhibited by the float valve. When the cement has set the top plug, bottom plug, float valve and residual cement are drilled out.
Typically, plug containers or cementing heads connected to the upper end of the casing string releasably hold cementing plugs until they are to be released ahead of and behind the cement as it is displaced through the cementing head into the well casing. Many prior art plug set systems are complex with many moving parts, some of which are exposed to the corrosive fluids flowing up and down in the wellbore. In cementing offshore wells drilled beneath a body of water, the plugs may be run into the wellbore with a casing string. A variety of problems are associated with such "sub sea" release systems; e.g. parts are eroded by sand, grit, and corrosive material in various fluids; positive indication of plug release is not achieved; plugs or parts of them are not made of easily drillable material; and ocean forces on casing extending from a drilling platform to a sub-sea wellhead bend and twist the casing, inhibiting or preventing the use of certain plugs.
This has led to the development of sub-sea cementing apparatus which generally comprises an open top plug and an open bottom plug which are releasably connected to one another. In use, the sub-sea cementing apparatus is positioned in the casing at or adjacent the sub-sea wellhead by a tool string. Circulating fluid is then pumped downwardly from the drilling platform through the tool string, the open top plug, the open bottom plug and the casing and flows upwardly through the annular space between the outside of the casing and the bore. This operation is typically carried out for several hours after which a first closure member, typically a ball or a dart, is dropped down the casing, passes through the top plug but closes the bottom plug. A required volume of cement is then pumped down from the drilling platform. This detaches the bottom plug from the top plug and forces the bottom plug to slide down the casing. Once the required volume of cement has been pumped into the casing a second closure member, typically a ball or a dart of larger diameter than the first dart is placed on the top of the cement and pumped down with drilling fluid. When the second closure member engages the top plug it closes the opening therein and further pressure from the drilling fluid releases the top plug down the casing. When the bottom plug engages the float valve at the bottom of the casing the pressure on the top plug is increased until a rupturable member in the bottom plug ruptures allowing the cement to pass through the float valve into the annular space between the outside of the casing and the bore. When the top plug engages the bottom plug the hydraulic pressure on the drilling fluid is released and the cement allowed to set after which the top plug, bottom plug, float valve and residual cement are drilled out.
The disadvantage with existing sub-sea equipment is that it has been extremely difficult to control the pressure at which the bottom plug is released and even more difficult to control the pressure at which the top plug is released. One very serious problem is when the pressure which has to be applied to release the bottom plug is so high that the top plug is simultaneously released thus severely delaying the cementing operation. Certain prior art sub-sea cementing apparatus is constructed primarily of aluminum and uses a multiplicity of shear pins to achieve release at desired pressures.
It is believed that aluminum is not the most suitable for certain sub-sea plug sets. Without wishing to be bound by any theory, the inventors believe that when existing sub-sea cement apparatus are maneuvered into position, relative movement between the parts of the apparatus causes small indentations in the surface of the aluminum which can form abutments which inhibit subsequent relative movement of parts at the desired pressure. Furthermore, the inventors believe that since, in practice, the fluid used during circulation often contains traces of sand and minute particles, these particles often become wedged between the parts of the apparatus, piercing or damaging the surface of the aluminum, and inhibiting relative movement of the parts.
Representative plug sets, plug containers, and release systems are shown in these U.S. Pat. Nos.: 5,392,852; 5,095,980; 5,004,048; 4,453,745; 4,433,859; 4,427,065; 4,290,482; 4,246,967; 4,164,980; 3,863,716; 3,635,288; 3,616,850; 3,545,542; and 2,620,037.